IL-2 is critical to immune regulation, and mice lacking IL-2 receptor subunits exhibit spontaneous autoimmune responses. T regulatory cells (TR) cells constitutively express IL-2 receptor signaling pathway receptors at high levels but do not express IL-2 when activated, suggesting that one function of these cells is to rapidly and efficiently respond to IL-2 produced during an immune response. We have shown that a patient lacking CD25, the high affinity IL-2 receptor, exhibits an autoimmune disease similar to patients lacking Foxp3. Analysis of PBMCs from this patient demonstrated a normal number of circulating Foxp3+ cells, suggesting that while IL-2 is not required for the survival of T regulatory (TR) cells, it is necessary for TR cell function. CD4 cells from this patient were unable to produce IL-10 in response to TCR activation and IL-2. We have shown that CD25 deficient mice exhibit autoimmune disease and defective IL-10 production from CD4 lymphocytes and TR cells, confirming our findings in humans. We hypothesize that TR cells respond to IL-2 produced by activated lymphocytes and produce IL-10 that limits the magnitude of the immune response. The broad, long term goals of this project are to understand the relationship of IL- 2 and IL-10 in immune regulation. We will use novel genetic models to clearly define the source of IL-10 required for immune regulation, and will determine the role of IL-2 in the production of IL-10. Specifically, this proposal will determine if TR cell CD25 expression is required for immune suppression, and this will be compared with TR cells from IL-10 deficient animals. We will determine the source of IL-10 using TR cells from mice expressing GFP under control of the IL-10 locus, and test whether CD25 deficient TR cells are able to produce IL-10 in vivo. Finally, we will generate IL-2 producing TR cells and test their suppressive function in models of immune activation. A well balanced immune system allows for the generation of an immune response to infectious agents while preventing attack of ones own tissues. These studies help to explain how the immune system maintains this balance. Once we understand how this balance is maintained, therapies can be developed to either strengthen our immunity to treat cancer or infections, or inhibit our immunity to treat autoimmune disease.